With rapid progress of wireless communication technology, mobile communication products have become the mainstream of modern science-and-technology products. These mobile communication products include a notebook computer, a cellular phone, and a personal digital assistant (PDA), etc. After coupling with the wireless communication modules, these products can link to the internet, receive and send electronic mails, and get instant information on news or stocks quotations so as to achieve functions of resource sharing and information transmitting.
A conventional “Printed Sleeve Antenna” disclosed by U.S. Pat. No. 5,598,174 relates to formation of a half wavelength resonant mechanism with extension of a ground strip to a quarter wavelength in an “L” shape and extension of a feed strip to a quarter wavelength so as to achieve effects similar to the traditional coaxial sleeve dipole. This conventional antenna design is concerned with single frequency transmission and cannot be applied in multi-frequency signal transmission. Moreover, the planar radiation field pattern is poor in omnidirectional performance due to the asymmetrical structure, and it is difficult to impedance match with a general symmetrical microstrip feeding. Furthermore, a conventional “Printed Antenna” disclosed by U.S. Pat. No. 5,754,145 relates to a printed dipole antenna with three printed strips to form a dipole mechanism so as to achieve effects similar to the traditional sleeve dipole. However, this antenna design is also concerned only with single frequency transmission.